Tsuki no Ko is a beautiful ~late 80's-early 90's shoujo manga that explores darker themes like reincarnation, the apocalyptic, alienation, loneliness, morality/moral obligation, and sacrifice rooted in profound love for another. While its mood is not necessarily dismal, the nature of the aforementioned themes lend it an implicit sense of melancholy that, in turn, runs throughout the bulk of every volume. That said, there are various breaks in this heaviness that approach a much lighter, almost slice-of-lifeish tone, as can be seen in some of the interactions between Art/Jimmy and often in Jimmy's behavior alone. These, I think, give the plot a nice balance and keep the reader from growing bored or disillusioned from a constant stream of drama.
The themes are orchestrated quite well and all have a secure place within the plot. They also aren't overly philosophical or abstracted, which allows the different character arcs to retain at least some sense of relatability, as well as prevents the manga itself from veering towards the labels of pretentious or navel-gazing. (note: I like abstract, philosophical works but I think their presence as characteristics in shoujo manga have their limits.)
Romance plays a pretty significant role in Tsuki no Ko, but its not the saccharine kind that is unfortunately present in many shoujos; the idea of being a star-crossed or jilted lover is heavily bound up in the story. The relationships are pretty eccentric, which is in accordance with the unusual nature of the story as a whole. I liked this aspect, but it requires some suspension of disbelief, which I guess you should expect if you're reading a supernatural shoujo manga.
There are some plot holes, but I don't think they're significant enough to hinder the reader's comprehension or serve as a detriment to the story. I'm not a fan of the ending, however, and did find it kind of rushed in comparison to pace of preceding chapters.
The art is consistently pretty and at times, beautiful. I didn't have any major qualms with it, but I am a fan of bishoujo and bishounen characters and the shoujo art style as a whole.
The personalities and motivations of the characters are, for the most part, fleshed out quite well. Each character had some sort of 'fatal flaw', which added some realism into an otherwise supernaturally based story. Both Art and Jimmy can be annoying, but I think that's necessary for their characterization.
The first volume made immersion a little difficult, but it became steadily better thereon. The introduction of Jimmy's siblings (I'm not sure what chapter/volume this happens in exactly, but its relatively early on) was, for me, when things got really intriguing.
My dislike of the ending, coupled with some minor plot holes, made me give this an 8. I was absolutely recommend it. I found it quite similar to Please Save My Earth, though not as sad.